Conventionally, a grain oriented electrical steel sheet has been used as iron core material for a transformer in which good magnetic properties are required. When using the grain oriented electrical steel sheet as the iron core material for the transformer, in order to reduce a loss of energy, among the magnetic properties, it is essential to reduce an iron loss.
For the purpose of reducing the iron loss, several efforts have been made including: a method of increasing an electric resistance of the steel sheet by means of increasing a silicon (Si) content, a method of precisely aligning a crystal orientation to (110) [001] orientation, and a method of making the steel sheet thinner to reduce a thickness of the steel sheet. However, a metallurgical method has itself a limit to reduce the iron loss.
Other effort to reduce the iron loss has been made including a method of refining a magnetic domain artificially. Patent Literature 1 discloses a method of refining the magnetic domain including a step of irradiating a laser onto a surface of the steel sheet on which finish annealing has been made. The method of the patent literature results in an effective reduction of the iron loss, however, it has a problem in that degradation of the iron loss is introduced because thermal strain induced by the laser refines the magnetic domain and thus strain relief annealing restores thermal strain. In a method of producing a steel sheet for a wound iron core, the strain relief annealing is essential step. Therefore, it is hard to employ the method disclosed in the Patent Literature 1 as a method of reducing an iron loss in the grain oriented electrical steel sheet.
Patent Literature 2 is known which discloses a method of producing a grain oriented electrical steel sheet including a strain relief annealing step as a technique capable of suppressing the degradation of the iron loss. The Patent Literature 2 discloses a method including steps of: coating a resist ink lineally to form a patterning; etching the steel sheet using the resultant resist pattern as a mask; and refining the magnetic domain by means of the formed linear grooves.
Generally, as a method of coating a resist ink in a linear pattern, an inkjet, a slot die coater, a gravure roll coater or the like has been utilized. When coating a base material such a steel sheet that is rigid and likely deformed, a rubber roll is suitable because the rubber roll enable itself to follow the deformation of the steel sheet due to elastic deformation.
The method of coating the resist ink using the rubber roll includes coating a resist ink on a surface of the steel sheet while pinching the steel sheet with an offset roll in which rubber is lined on a steel roll and a backup roll on which steel sheet is wound. Because the resist ink is coated on the steel sheet with being wound on the backup roll, a form of the steel sheet is corrected in a smooth form to some extent, so that the resist pattern is also smoothly coated.